The origin of this is rumoured to be that when the ticket seller/doorman carnie noticed that a visitor was carrying a lot of cash in his wallet, he would pat him on the back while palming a piece of chalk, leaving a mark on the wealthy visitor's back. Thus the pickpockets knew who to target.
Now understand this my nigga Dre can't be touched
Luke's bendin over, so Luke's gettin fucked, busta
Musta, thought I was sleazy
Or thought I was a mark cause I used to hang with Eazy